Modems that block data transfers during safe mode of operation and related methods

ABSTRACT

A cable modem blocks data transfers addressed to host systems during a safe mode of operation and provides the data transfers to the addressed host systems during a normal mode of operation. The cable modem can be coupled to multiple host systems which can be addressed separately. Accordingly, the safe mode of operation can be selected for a subset of the host systems coupled to the cable modem. The safe mode or normal mode of operation can be invoked by pushing a button on the cable modem. The safe mode of operation may also be invoked after the expiration of a timer which times an interval of inactivity at the host. The normal mode of operations can be invoked after detecting activity at the host system.

FIELD OF THE INVENTION

The present invention relates to the field of communications in generaland more particularly to cable modems and methods.

BACKGROUND OF THE INVENTION

With the rise in popularity of the Internet, many users are accessingthe Internet through the Public Switched Telephone Network (PSTN) over amodem connected to a telephone line in the user's home. Unfortunately,the bandwidth provided by home telephone lines may prove to beinadequate for some applications on the Internet. For example, some datasets provided by the Internet may be so large that it is difficult totransfer the data set over the telephone line in a given time so thatthe application operates in a real-time manner. In particular, currentresidential telephone modem technology may be limited to data rates onthe order of 56 kilobaud (kb).

In an attempt to reduce the bandwidth problem associated with thetelephone lines described above, there have been efforts to provideInternet service over coaxial cables used to provide cable TV.Accordingly, the user may access the Internet over the cable systemusing a cable modem to provide data rates of 42 megabaud or higher.Accessing the Internet via a cable system may involve initializing thecable modem each time the cable modem is turned on, during which thecable modem may register with the cable system. For example, when theuser wishes to access the Internet, the user may turn on the cable modemwhich then registers with the cable system.

As the number of cable modems handled by the cable system increases, thetime needed to register each cable modem may also increase therebylengthening the registration time. For example, if hundreds of cablemodems are used in a cable system, the registration time for a selectedcable modem may be several minutes. Consequently, the user may wish toavoid turning the cable modem off in an effort to avoid the delayincurred by a lengthy registration process. For example, if the userturns the cable modem on just prior to accessing the Internet, the usermay need to wait for the registration process to complete before gainingaccess to the Internet. Moreover, cable systems may also providetelevision and telephone service to a user's home such as by routingthese services through the cable modem to the television and telephone.Accordingly, the user may desire that the cable modem be left on so asnot to interrupt telephone or television service.

Unfortunately, leaving the cable modem turned on may decrease thesecurity of the computer to which the cable modem is attached. Inparticular, the computer may be more susceptible to attack via thecable. For example, an unauthorized user may attempt to gain access tothe computer via the cable. Moreover, because the cable providesrelatively high bandwidth, relatively simple attacks, such as trying alarge number of password combinations, may require only a short time tobe successful. In view of the above, there exists a need to improve thesecurity of cable modems used to access the Internet via cable systems.

Accordingly, the present invention may allow improvement in the securityof cable modems by blocking access to the cable modem from the cablesystem while the cable modem is in safe mode. Blocking data transfersmay allow the subscriber to leave the host system connected to the cablemodem, thereby possibly avoiding the delay associated with theregistration process while reducing the security threats posed bymaintaining a physical connection to the cable modem.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to allowimprovement in cable modems and methods.

It is another object of the present invention to allow improvement inthe security of able modems and methods.

These and other objects are provided by blocking data transfers from acable system to a cable modem addressed to a host system coupled to thecable modem during a safe mode of operation and providing the datatransferred from the cable system to the addressed host system during anormal mode of operation. Blocking the data transfers addressed to ahost system during the safe mode of operation may improve the securityof the cable modem. In particular, while the cable modem operates in thesafe mode of operation, unauthorized users will not be able to gainaccess to the host system via the cable system. Moreover, other servicesprovided via the cable modem can be available while the cable modemoperates in the safe mode. For example, telephone or television servicesmay be unaffected by the safe mode of operation. Subscribers may,therefore, leave the cable modem and host turned on to avoid a lengthyregistration process while reducing the likelihood of unauthorizedaccess to the host system.

In another aspect of the present invention, multiple host systems can becoupled to the cable modem. Accordingly, the cable modem can provide asafe mode of operation for each host system coupled to the cable modem.In particular, the data transfer can include an address portion, andeach of the host systems can be identified by a respective host address.Data transfers can be addressed to a selected host system using therespective host address. Therefore, data transfers addressed to a firsthost system can be blocked when the cable modem operates in a first safemode of operation for the first host system, and data transfersaddressed to a second host system can be blocked when the cable modemoperates in a second safe mode of operation for the second host system.

In a further aspect of the present invention, the cable modem caninclude a safe mode button wherein the safe mode of operation is invokedupon pushing the safe mode button. In another embodiment, the safe modeof operation can be invoked after the expiration of a timer that timesan interval of inactivity at the host system. In a further embodiment,the cable modem can switch from the safe mode of operation to the normalmode of operation after activity is detected at the host system. Instill another embodiment, the mode of operation is invoked on receipt ofa command from the host system.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram of an embodiment of a cable system accordingto the present invention;

FIG. 2 is a block diagram of an embodiment of the cable modem 100 ofFIG. 1; and

FIG. 3 is a flowchart that illustrates operations of a cable modemaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

As will be appreciated by one of skill in the art, the present inventionmay be embodied as methods or devices. Accordingly, the presentinvention may take the form of an entirely hardware embodiment, anentirely software embodiment or an embodiment combining software andhardware aspects.

The present invention is also described using a flow chart illustration.It will be understood that each block of the flowchart illustration, andcombinations of blocks in the flowchart illustration, can be implementedby computer program instructions. These program instructions may beprovided to a processor within the cable modem, such that theinstructions which execute on the processor create means forimplementing the functions specified in the flowchart block or blocks.The computer program instructions may be executed by the processor tocause a series of operational steps to be performed by the processor toproduce a computer implemented process such that the instructions whichexecute on the processor provide steps for implementing the functionsspecified in the flowchart block or blocks.

Accordingly, blocks of the flowchart illustration support combinationsof means for performing the specified functions, combinations of stepsfor performing the specified functions and program instruction means forperforming the specified functions. It will also be understood that eachblock of the flowchart illustration, and combinations of blocks in theflowchart illustration, can be implemented by special purposehardware-based systems which perform the specified functions or steps,or combinations of special purpose hardware and computer instructions.It will be understood that the term “coupled” as used herein to describearrangements of devices includes arrangements wherein interveningdevices are present between the coupled devices. For example, where afirst device is described as coupled to a second device, the descriptionwill be understood to include other devices located between and coupledto the first and second devices.

FIG. 1 is a block diagram of an embodiment of a cable system accordingto the present invention. The cable system includes a Cable ModemTermination System or Cable Modem Terminal Server (CMTS) 140 and aplurality of cable modems 100. The CMTS 140 can provide services, suchas television service, telephone service, and internet service, tosubscribers of the cable system via cable modems 100 by transferringdata over a cable 110, such as a coaxial cable. For example, asubscriber may access the Internet through the respective cable modem100 from a host 125 such as a Personal Computer (PC).

The CMTS 140 manages the services provided to the respective subscribersin the cable system. For example, a first subscriber may receivetelevision, telephone, and Internet services while a second subscribermay receive only Internet service. Moreover, different subscribers mayreceive a different quality of service. For example, a first subscribermay receive Internet service at relatively low bandwidth while a secondsubscriber may receive Internet service at relatively high bandwidth.Accordingly, the CMTS 140 transmits and receives data to and from therespective cable modems 100 a-f at the rates associated with therespective subscribers. The CMTS 140 can also adjust parameters of thecable modems 100 a-f used to transfer data such as phase timing,frequencies and power levels associated with the transfer of databetween the respective cable modems 100 a-f and the CMTS 140. Forexample, the CMTS 140 can monitor the timing and power levels of thedata transferred from the respective cable modems 100 a-f and instructeach cable modems 100 a-f to adjust the timing and power level of thedata transfer performed by the cable modems 100 a-f.

In one embodiment, the Internet service provided by the CMTS 140includes data transferred between the CMTS 140 and the cable modem 100 avia the cable 110 at respective frequencies. For example, the subscribermay request information from the Internet, wherein data is transferredfrom the host 125 through the cable modem 100 a to the CMTS 140 over thecable 110 at a first frequency. The CMTS 140 responds to the request forinformation by transferring the requested data from the CMTS 140 to thehost 125 through the cable modem 100 a over the cable 110 at a secondfrequency. In another embodiment, the request is transmitted via atelephone line which is not part of the cable system.

It will be understood by those of skill in the art, that the datatransfers between the CMTS 140 and the cable modem 100 a may beperformed according to standards known in the art. For example, datatransfers between the CMTS 140 and the cable modem 100 a may beperformed using a Time Division Multiple Access (TDMA) technique whereindata is transmitted and received over the cable 110 in predefinedtime-slots. Standards for the transfer of data in cable systems arediscussed in the Data Over Cable System Interface Specification(DOCSIS).

When the cable modem 100 a is turned on, the cable modem 100 a performsan initialization sequence wherein the cable modem 100 a registers withthe cable system. In particular, the cable modem 100 a transfers anidentifier to the CMTS 140 that identifies the cable modem 100 a withinthe cable system. Accordingly, the CMTS 140 can communicate with theselected cable modem 100 a using the cable modem's respectiveidentifier.

During registration, the CMTS 140 performs ranging of each of the cablemodems 100 a-f registered to adjust for the propagation delay of thatdata transferred, to adjust the proper power level of the data transfer,and to determine the quality of service provided to the subscriber. Iftelephone service is provided to the subscriber via the cable system,the registration process can also include the determination ofparameters for the telephone service.

After registration, the CMTS 140 provides services to the subscriber viathe respective cable modem 100 a. In operation, services are provided bydata transfers between the CMTS 140 and the cable modem 100 a. Inparticular, data is transferred from the CMTS 140 to a selected cablemodem 100 a using the identifier that identifies the selected cablemodem 100 a in the cable system. In operation, the data transfer to theselected cable modem 100 a, includes the identifier which matches theidentifier of the selected cable modem 100 a. For example, if theselected cable modem 100 a has an associated identifier of 800, a datatransfer including an identifier of 800 will be accepted by the selectedcable modem 100 a. A data transfer can include information and/or acommand directed to the selected cable modem 100 a.

FIG. 2 is a block diagram of an embodiment of the cable modem 100 ofFIG. 1. Data is transferred between the CMTS 140 and the cable modem 100by a Media Access Controller (MAC) 105 coupled to the cable 110. The MAC105 accepts data transfers from the CMTS 140 if the identifier includedin the data transfer matches the identifier of the cable modem 100. Thedata transfer can include information intended for a first host 125, asecond host 130, a telephone 107, the cable modem 100, or other deviceaccessed via the cable modem 100. For example, the data transfer mayinclude information intended for the first host 125 in response to arequest made by the first host 125 or a range command for the cablemodem 100 to transfer timed information to the CMTS 140. The functionsprovided by the MAC 105 may be provided by software running on processor115 or by hardware and/or software separate from the processor. Whilethe processor, media access controller, host interface controller, andtelephone interface controller of FIG. 2 are illustrated as separateblocks, it will be understood that one or more of these portions of themodem or sub-portions thereof, can be implemented using combinedhardware and/or software.

The data transfer may include an address specifying which device coupledto the cable modem 100 is the destination of the data transfer. Forexample, if the data transfer is intended for the second host 130, theaddress included in the data transfer identifies the second host 130 asthe destination. Although not shown, the MAC 105 may be coupled to aRadio Frequency (RF) tuner that modulates and demodulates the dataincluded in the data transfers. For example, data transfers to the CMTS140 may be modulated and transferred over a first channel on the cable110. The RF tuner demodulates the data transferred from the CMTS 140over a second channel on the cable 110.

A processor 115 coordinates operations of the cable modem 100 within thecable system to provide the selected services to the subscriber.According to the present invention, data transfers to addressed hostsare blocked by the processor 115 during a safe mode of operation and notblocked by the processor 115 during a normal mode of operation.Moreover, blocking can be performed on a per host basis. For example, inone embodiment, data transfers addressed to the first host 125 areblocked while data transfers addressed to the second host 130 arereceived and provided to the second host 130. Moreover, data transfersincluding commands for the cable modem 100 or addressed to devices otherthan the hosts are unaffected by the safe mode of operation. Forexample, a ranging command issued to the cable modem 100 during safemode of operation is accepted and responded to by the MAC 105.

In one embodiment, the safe or normal mode of operation is selectedusing software that maintains a safe mode flag that is set to one of asafe mode state or a normal state flag to indicate the selected mode ofoperation. For example, the flag can be set to the safe mode state toindicate the safe mode of operation and set to the normal mode state toindicate the normal mode of operation. In one embodiment, the mode ofoperation is selected by pressing a safe mode button 108 on the housingof the cable modem 100 a. The safe mode button 108 can be a momentaryswitch that causes the processor 115 to toggle the mode of operation.

In another embodiment, the mode of operation is selected via a commandissued by the host. For example, the subscriber may cause a command tobe issued to the cable modem 100 whereupon the cable modem 100 a changesthe mode of operation. In still another embodiment, the mode ofoperation is selected based on a level of activity at the host. Forexample, the safe mode of operation can be selected after a period ofinactivity at the first host 125 is observed over a predetermined timeinterval. The cable modem 100 a can resume the normal mode of operationupon the resumption of activity at the first host 125. Accordingly, themode of operation can be selected based on the subscriber's use of thehost coupled to the cable modem 100 a. Moreover, the cable modem 100 aneed not be located near the subscriber for the mode of operation to beselected. For example, the cable modem 100 may be located in thebasement of the subscriber's home while the host is located in thesubscriber's home office.

A Host Interface Controller (HIC) 135 provides the data received by theprocessor 115 to the addressed host and provides data from the host tothe processor 115 for transfer to the CMTS 140. The HIC 135 can be acontroller suitable for interfacing to at least one host, such as anEthernet controller, Universal Serial Bus (USB) or other type ofinterface known to those of skill in the art.

A telephone interface 116 provides telephone data from a telephone 107,such a Data Telephone Equipment (DTE), to the processor 115 and providesdata transferred from the CMTS 140 to the telephone 107. As describedabove, data transfers to the telephone 107 from the CMTS 140 areunaffected by safe mode of operation.

FIG. 3 is a flowchart illustrating operations of a cable modem 100according to the present invention. According to FIG. 3, the cable modem100 performs initialization upon being turned on or reset (block 300).The processor 115 reads the stored value of the safe mode flag todetermine which mode of operation is selected (block 305) and resets ahost inactivity timer that indicates the elapsed time since hostactivity was last detected.

If the safe mode flag indicates that safe mode of operation is selected(block 310) for host 115, the processor determines if data transfersfrom the CMTS 140 to host 125 are currently enabled (block 320). If datatransfers are not enabled (block 320) to the host 125, the processor 115waits for host activity to be detected (block 340) at host 125.Otherwise, if data transfers are currently enabled (block 320) to host125, the processor 115 disables data transfers (block 335) and thenwaits for host activity at host 125 to be detected (block 340).

The cable modem 100 continues to operate in the safe mode of operationuntil host activity is detected at host 125 whereupon the safe mode flagis cleared and the host inactivity timer is reset (block 345), or untila safe mode button is pushed (block 350) thereby changing the safe modeof operation to the normal mode of operation (block 360) and resettingthe host inactivity timer.

When the normal mode of operation is enabled (block 310), due to thecommencement of activity at the host 125 (block 345) or by pressing thesafe mode button (block 360), the processor determines if data transfersfrom the CMTS 140 are currently disabled (block 315). If data transfersare disabled (block 315), the processor 115 enables data transfers andwaits for the host inactivity timer to expire (block 330). Otherwise theprocessor 115 waits for the host inactivity timer to expire (block 330).

The cable modem 100 continues to operate in the normal mode of operationuntil the host inactivity timer expires (block 330) whereupon the safemode flag is set, or until the safe mode button is pushed (block 350)thereby changing the normal mode of operation to safe mode of operation(block 360).

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms areemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being set forthin the following claims.

What is claimed is:
 1. A cable modem that transfers data between a cablesystem and a host system via a cable, the cable modem comprising: amedia access controller, coupled to the cable, wherein the media accesscontroller accepts data from the cable system via the cable; aprocessor, coupled to the media access controller, wherein the processorreceives the data accepted from the cable system for the host system toprovide received host data during a normal mode of operation and whereinthe processor blocks data accepted from the cable system for the hostsystem during a safe mode of operation; and a host interface, coupledbetween the processor and the host system, wherein the host interfaceprovides the received host data to the host system during the normalmode of operation.
 2. The cable modem of claim 1: wherein the hostinterface is coupled between the processor and a second host system;wherein the first and second host systems are identified by respectivefirst and second host addresses; wherein the received host data includesan address portion; wherein the host interface provides the receivedhost data to the first host system when the address portion of thereceived host data includes the first host address; and wherein the hostinterface provides the received host data to the second host system whenthe address portion of the received host data includes the second hostaddress.
 3. The cable modem of claim 2, wherein the processor blocksdata accepted from the cable system for both the first and second hostsystems during the safe mode of operation.
 4. The cable modem of claim 2wherein the safe mode of operation comprises a first safe mode ofoperation for the first host system and a second safe mode of operationfor the second host system, wherein data accepted from the cable systemaddressed to the first host system is blocked during the first safe modeof operation for the first host system, and wherein data accepted fromthe cable system addressed to the second host system is blocked duringthe second safe mode of operation for the second host system.
 5. Thecable modem of claim 1 further comprising: a cable modem housing; and asafe mode button, wherein the safe mode of operation is invoked uponpressing the safe mode button.
 6. The cable modem of claim 1, whereinthe processor invokes the safe mode of operation after expiration of atime interval during which no activity is detected at the host system.7. The cable modem of claim 1, wherein the processor invokes the normalmode of operation after detecting activity at the host system.
 8. Thecable modem of claim 1, wherein the safe mode of operation is invoked onreceipt of a safe mode command from the host system.
 9. The cable modemof claim 1, wherein the processor includes a safe mode flag that can beset by the processor to one of a safe mode state and a normal modestate, wherein the safe mode of operation is invoked when the safe modeflag is set to the safe mode state.
 10. The cable modem of claim 9,wherein the media access controller processes addresses included in thedata during the safe mode of operation and during the normal mode ofoperation, wherein the cable modem further comprises: a telephoneinterface, coupled to the processor, wherein the telephone interfaceprovides telephone data external to the cable modem during the safe modeof operation and during the normal mode.
 11. A method of transferringdata between a cable system and a first host system, the methodcomprising the steps of: accepting data from the cable system; receivingthe data accepted from the cable system for the host system to providereceived host data during a normal mode of operation; blocking the dataaccepted from the cable system for the host system during a safe mode ofoperation; and providing the received host data to the first host systemduring the normal mode of operation.
 12. The method of claim 11, whereinthe data is transferred between the cable system, the first host system,and a second host system, wherein the first and second host systems areidentified by respective first and second host addresses, and whereinthe received host data includes an address portion, wherein the step ofproviding comprise the steps of: providing the received host data to thefirst host system when the address portion of the received host dataincludes the first host address; and providing the received host data tothe second host system when the address portion of the received hostdata includes the second host address.
 13. The method of claim 12,wherein the step of blocking comprises the step of blocking dataaccepted from the cable system addressed to both the first and secondhost systems during the safe mode of operation.
 14. The method of claim12, wherein the safe mode of operation comprises a first safe mode ofoperation for the first host system and a second safe mode of operationfor the second host system, wherein the step of blocking comprises thestep of blocking the data accepted from the cable system addressed tothe first host system during the first safe mode of operation for thefirst host system and blocking the data accepted from the cable systemaddressed to the second host system during the second safe mode ofoperation for the second host system.
 15. The method claim 11, whereinthe safe mode of operation is invoked upon detecting activation of asafe mode button.
 16. The method of claim 11, wherein the step ofblocking comprises the step of invoking the safe mode of operation afterexpiration of a time interval during which no activity at the first hostsystem is detected.
 17. The method of claim 11, wherein the step ofproviding comprises the step of invoking the normal mode of operationafter detecting activity at the first host system.
 18. The method ofclaim 11, wherein the step of blocking comprises the step of blockingthe data accepted from the cable system during the safe mode ofoperation, wherein the safe mode of operation is invoked on receipt of asafe mode command from the first host system.
 19. The method of claim11, wherein a safe mode flag is set to a safe mode state to indicatethat the safe mode of operation is enabled or to a normal mode state toindicate that the normal mode of operation is enabled.
 20. A cable modemthat transfers data between a cable system and a first host system via acable, the cable modem comprising: means for accepting data from thecable system; means for receiving the data accepted from the cablesystem to provide received host data during a normal mode of operation;means for blocking the data accepted from the cable system during a safemode of operation; and means for providing the received host data to thefirst host system during the normal mode of operation.
 21. The cablemodem of claim 20, wherein the data is transferred between the cablesystem, the first host system, and a second host system, wherein thefirst and second host systems are identified by respective first andsecond host addresses, wherein the received host data includes anaddress portion, wherein the means for providing comprises: means forproviding the received host data to the first host system when theaddress portion of the received host data includes the first hostaddress; and means for providing the received host data to the secondhost system when the address portion of the received host data includesthe second host address.
 22. The cable modem of claim 21, wherein themeans for blocking comprises means for blocking data accepted from thecable system addressed to both the first and second host systems duringthe safe mode of operation.
 23. The cable modem of claim 21, wherein thesafe mode of operation comprises a first safe mode of operation for thefirst host system and a second safe mode of operation for the secondhost system, wherein the means for blocking comprises means for blockingthe data accepted from the cable system addressed to the first hostsystem during the first safe mode of operation for the first host systemand means for blocking the data accepted from the cable system addressedto the second host system during the second safe mode of operation forthe second host system.
 24. The cable modem claim 20, wherein the meansfor blocking comprises a safe mode button, wherein the safe mode ofoperation is invoked upon pressing the safe mode button.
 25. The cablemodem of claim 20, wherein the means for blocking comprises means forinvoking the safe mode of operation after expiration of a time intervalduring which no activity at the first host system is detected.
 26. Thecable modem of claim 20, wherein means for providing comprises means forinvoking the normal mode of operation after detecting activity at thefirst host system.
 27. The cable modem of claim 20, wherein the meansfor blocking comprises means for blocking the data accepted from thecable system during the safe mode of operation, wherein the safe mode ofoperation is invoked on receipt of a safe mode command from the firsthost system.
 28. The cable modem of claim 20, wherein a safe mode flagis set to indicate that the safe mode of operation is enabled and iscleared to indicate that the normal mode of operation is enabled.
 29. Amodem that transfers data between a central system and a host system viaa transmission medium, the modem comprising: a media access controller,coupled to the transmission medium, wherein the media access controlleraccepts data from the central system via the transmission medium; aprocessor, coupled to the media access controller, wherein the processorreceives the data accepted from the central system for the host systemto provide received host data during a normal mode of operation andwherein the processor blocks data accepted from the central system forthe host system during a safe mode of operation; and a host interface,coupled between the processor and the host system, wherein the hostinterface provides the received host data to the host system during thenormal mode of operation.
 30. The modem of claim 29: wherein the hostinterface is coupled between the processor and a second host system;wherein the first and second host systems are identified by respectivefirst and second host addresses; wherein the received host data includesan address portion; wherein the host interface provides the receivedhost data to the first host system when the address portion of thereceived host data includes the first host address; and wherein the hostinterface provides the received host data to the second host system whenthe address portion of the received host data includes the second hostaddress.
 31. The modem of claim 30, wherein the processor blocks dataaccepted from the central system for both the first and second hostsystems during the safe mode of operation.
 32. The modem of claim 30,wherein the safe mode of operation comprises a first safe mode ofoperation for the first host system and a second safe mode of operationfor the second host system, wherein data accepted from the centralsystem addressed to the first host system is blocked during the firstsafe mode of operation for the first host system, and wherein dataaccepted from the central system addressed to the second host system isblocked during the second safe mode of operation for the second hostsystem.
 33. The modem of claim 29, further comprising: a modem housing;and a safe mode button, wherein the safe mode of operation is invokedupon pressing the safe mode button.
 34. The modem of claim 29, whereinthe processor invokes the safe mode of operation after expiration of atime interval during which no activity is detected at the host system.35. The modem of claim 29, wherein the processor invokes the normal modeof operation after detecting activity at the host system.
 36. The modemof claim 29, wherein the safe mode of operation is invoked on receipt ofa safe mode command from the host system.
 37. The modem of claim 29,wherein the processor includes a safe mode flag that can be set by theprocessor to one of a safe mode state and a normal mode state, whereinthe safe mode of operation is invoked when the safe mode flag is set tothe safe mode state.
 38. The modem of claim 37, wherein the media accesscontroller processes addresses included in the data during the safe modeof operation and during the normal mode of operation, wherein the modemfurther comprises: a telephone interface, coupled to the processor,wherein the telephone interface provides telephone data external to themodem during the safe mode of operation and during the normal mode. 39.A method of transferring data between a central system and a first hostsystem, the method comprising the steps of: accepting data from thecentral system; receiving the data accepted from the central system forthe host system to provide received host data during a normal mode ofoperation; blocking the data accepted from the central system for thehost system during a safe mode of operation; and providing the receivedhost data to the first host system during the normal mode of operation.40. The method of claim 39, wherein the data is transferred between thecentral system, the first host system, and a second host system, whereinthe first and second host systems are identified by respective first andsecond host addresses, and wherein the received host data includes anaddress portion, wherein the step of providing comprise the steps of:providing the received host data to the first host system when theaddress portion of the received host data includes the first hostaddress; and providing the received host data to the second host systemwhen the address portion of the received host data includes the secondhost address.
 41. The method of claim 40, wherein the step of blockingcomprises the step of blocking data accepted from the central systemaddressed to both the first and second host systems during the safe modeof operation.
 42. The method of claim 40, wherein the safe mode ofoperation comprises a first safe mode of operation for the first hostsystem and a second safe mode of operation for the second host system,wherein the step of blocking comprises the step of blocking the dataaccepted from the central system addressed to the first host systemduring the first safe mode of operation for the first host system andblocking the data accepted from the central system addressed to thesecond host system during the second safe mode of operation for thesecond host system.
 43. The method claim 39, wherein the safe mode ofoperation is invoked upon detecting activation of a safe mode button.44. The method of claim 39, wherein the step of blocking comprises thestep of invoking the safe mode of operation after expiration of a timeinterval during which no activity at the first host system is detected.45. The method of claim 39, wherein the step of providing comprises thestep of invoking the normal mode of operation after detecting activityat the first host system.
 46. The method of claim 39, wherein the stepof blocking comprises the step of blocking the data accepted from thecentral system during the safe mode of operation, wherein the safe modeof operation is invoked on receipt of a safe mode command from the firsthost system.
 47. The method of claim 39, wherein a safe mode flag is setto a safe mode state to indicate that the safe mode of operation isenabled or to a normal mode state to indicate that the normal mode ofoperation is enabled.
 48. A modem that transfers data between a centralsystem and a first host system via a transmission medium, the modemcomprising: means for accepting data from the central system; means forreceiving the data accepted from the central system to provide receivedhost data during a normal mode of operation; means for blocking the dataaccepted from the central system during a safe mode of operation; andmeans for providing the received host data to the first host systemduring the normal mode of operation.
 49. The modem of claim 48, whereinthe data is transferred between the central system, the first hostsystem, and a second host system, wherein the first and second hostsystems are identified by respective first and second host addresses,wherein the received host data includes an address portion, wherein themeans for providing comprises: means for providing the received hostdata to the first host system when the address portion of the receivedhost data includes the first host address; and means for providing thereceived host data to the second host system when the address portion ofthe received host data includes the second host address.
 50. The modemof claim 49, wherein the means for blocking comprises means for blockingdata accepted from the central system addressed to both the first andsecond host systems during the safe mode of operation.
 51. The modem ofclaim 49, wherein the safe mode of operation comprises a first safe modeof operation for the first host system and a second safe mode ofoperation for the second host system, wherein the means for blockingcomprises means for blocking the data accepted from the central systemaddressed to the first host system during the first safe mode ofoperation for the first host system and means for blocking the dataaccepted from the central system addressed to the second host systemduring the second safe mode of operation for the second host system. 52.The modem claim 48, wherein the means for blocking comprises a safe modebutton, wherein the safe mode of operation is invoked upon pressing thesafe mode button.
 53. The modem of claim 48, wherein the means forblocking comprises means for invoking the safe mode of operation afterexpiration of a time interval during which no activity at the first hostsystem is detected.
 54. The modem of claim 48, wherein means forproviding comprises means for invoking the normal mode of operationafter detecting activity at the first host system.
 55. The modem ofclaim 48, wherein the means for blocking comprises means for blockingthe data accepted from the central system during the safe mode ofoperation, wherein the safe mode of operation is invoked on receipt of asafe mode command from the first host system.
 56. The modem of claim 48,wherein a safe mode flag is set to indicate that the safe mode ofoperation is enabled and is cleared to indicate that the normal mode ofoperation is enabled.